JPWO2014185178A1 - Heat storage system - Google Patents

Abstract

Provided is a heat storage system capable of suppressing a decrease in heat transfer due to a heat storage material in a solid phase. In the heat storage system in which the minimum heat storage temperature is Tmin, a heat storage material made of a mixed salt of a non-eutectic composition that is in a solid-liquid coexistence state at the minimum heat storage temperature Tmin is used.

Description

  The present invention relates to a heat storage system.

  A solar thermal power generation system is known in which sunlight is collected and collected in a heat collection area, and steam is generated by this heat to drive a steam turbine to generate electric power. In a solar thermal power generation system, a heat storage system is generally provided in order to supplement power generation at night or in a time zone where the amount of solar radiation cannot be obtained and to suppress a transient change in output power.

As a heat storage system used for solar thermal power generation systems, etc., a molten salt called liquid solar salt (mixture of potassium nitrate (KNO ₃ ) and sodium nitrate (NaNO ₃ ), mass fraction of sodium nitrate is 0.6) is a heat storage material It is known that the heat is stored by sensible heat (for example, see Patent Document 1).

  However, when the heat storage amount becomes as large as 1 TJ, for example, the heat storage tank using the sensible heat becomes too large, which is not realistic. Therefore, in order to make the size of the heat storage tank realistic, a heat storage system using latent heat is desired.

Japanese Patent Laid-Open No. 5-256591

  However, the conventional heat storage system using latent heat has a problem that the heat storage material that has become a solid phase at the time of solidification adheres to the heat transfer tube, causing a decrease in heat transfer.

In order to solve this problem, it is conceivable to spread the heat transfer tubes throughout the heat storage tank to promote heat transfer. However, if the heat storage tank is large, it is The cost will be high. In addition, by using latent heat, it is possible to reduce the size of the heat storage tank as compared with the case of using sensible heat, but even in this case, the volume of the heat storage tank is 1000 m ³ when the heat storage amount is about 1 TJ. It is not easy to stretch the heat transfer tubes around the entire heat storage tank.

  Then, this invention solves the said subject, and aims at providing the thermal storage system which can suppress the heat-transfer fall by the thermal storage material used as the solid phase in the thermal storage system using a latent heat.

The present invention has been made in order to achieve the above object, the heat storage minimum temperature in the heat storage system is a T _min, as a heat storage material, the heat storage minimum temperature T becomes the solid-liquid coexistence state in _min of non-eutectic composition This is a heat storage system using a mixed salt.

  As the heat storage material, a material having a temperature range in which a solid-liquid coexistence state is 1 ° C. or more may be used.

The minimum heat storage temperature T _min may be 150 ° C. or higher.

  ADVANTAGE OF THE INVENTION According to this invention, the thermal storage system which can suppress the heat-transfer fall by the thermal storage material used as the solid phase can be provided.

(A) is a schematic diagram of a thermal storage system according to an embodiment of the present invention, (b) is a state transition diagram of a mixture of KNO ₃ and NaNO ₃ is used as the heat storage material in the present invention. It is a state transition diagram of a mixture of CsNO ₃ and NaNO ₃ used as a heat storage material in the present invention. Is a state transition diagram of a mixture of LiNO ₃ and NaNO ₃ is used as the heat storage material in the present invention. It is a state transition diagram of a mixture of NaNO ₃ and RbNO ₃ used as a heat storage material in the present invention. Is a state transition diagram of a mixture of LiBr and NaNO ₃ is used as the heat storage material in the present invention. It is a schematic diagram of the experimental apparatus used in the Example of this invention. In the Example of this invention, it is a graph which shows the temperature history of a thermal storage material and a metal plate, When (a) uses the thing of a non-eutectic composition as a thermal storage material, (b) is an eutectic composition as a thermal storage material. The case where the thing of is used is shown.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1A is a schematic diagram of a heat storage system according to the present embodiment, and FIG. 1B is a diagram of potassium nitrate (KNO ₃ ) and sodium nitrate (NaNO ₃ ) used as heat storage materials in the present embodiment. It is a state transition diagram of a mixture.

  As shown in FIG. 1A, the heat storage system 1 includes a heat storage tank 3 that stores the heat storage material 2 and two heat exchangers 4 and 5 provided in the heat storage tank 3.

  The cooling-side heat exchanger 4 is supplied with a low-temperature first heat medium, heat-exchanges the first heat medium and the heat storage material 2, thereby cooling the heat storage material 2, and the heat-exchanged heat exchanger 4 is heated to a high temperature. The heat medium is output to a heat load or the like. The cooling side heat exchanger 4 is composed of a heat transfer tube through which the first heat medium passes, and is provided only in the upper part in the heat storage tank 3.

  The heating-side heat exchanger 5 is supplied with a high-temperature second heat medium and heat-exchanges the second heat medium and the heat storage material 2 to heat and store the heat storage material 2. The heating side heat exchanger 5 includes a hot plate that is a plate-like heat exchanger in which a flow path for passing the second heat medium is provided, and is provided on the bottom surface of the heat storage tank 3.

  Although not shown, the heat storage tank 3 may be provided with stirring means for stirring the heat storage material 2. Further, the heat storage tank 3 may be provided with means for forcibly removing the heat storage material 2 in the solid phase from the surface of the cooling side heat exchanger 4.

In this embodiment, since it is assumed that the present invention is applied to a high-temperature system such as a solar thermal power generation system, the minimum heat storage temperature T _min is 150 ° C. or higher, preferably 200 ° C. or higher. Here, the case where the heat storage maximum temperature _Tmax of the heat storage system 1 is 400 degreeC and the heat storage minimum temperature _Tmin is 250 _degreeC is demonstrated as an example.

Now, in the heat storage system 1 according to this embodiment, as a heat storage material 2, used as comprising a mixed salt of a non-eutectic composition as the solid-liquid coexisting state at the heat storage minimum temperature T _min.

Here, a case where a binary mixed salt in which potassium nitrate (KNO ₃ ) and sodium nitrate (NaNO ₃ ) are mixed in a non-eutectic composition is used as the heat storage material 2 will be described.

As shown in FIG.1 (b), in the mixture of potassium nitrate and sodium nitrate, it becomes a eutectic with the composition in which the molar fraction of sodium nitrate becomes 0.49. Therefore, it is necessary to determine the composition so as to be in a solid-liquid coexistence state at 250 ° C., which is the lowest heat storage temperature T _min , and a composition other than the composition that becomes eutectic.

  In the present embodiment, the composition indicated by the thick broken line in FIG. 1B, that is, the composition having a sodium nitrate molar fraction of 0.786 (mass fraction of 0.755) is used as the heat storage material 2. It was.

In this case, when the heat storage material 2 is gradually cooled from the heat storage maximum temperature T _max of 400 ° C., a solid phase is generated at 274 ° C., and when further cooled, the heat storage minimum is increased while gradually increasing the solid phase rate. It reaches a temperature T _min of 250 ° C. At this time, a solid phase is generated at the coldest cooling surface (the surface of the heat transfer tube as the cooling side heat exchanger 4), and the solid phase grows along the cooling surface. Since it does not adhere, it peels off due to the flow of the heat storage material 2 or the like. In addition, if it cools further from 250 _degreeC which is the heat storage minimum temperature _Tmin , the heat storage material 2 will be in a solid phase completely at 234 degreeC. That is, the temperature region indicated by the bold solid line in the figure is in a solid-liquid coexistence state.

  The solid phase rate of the heat storage material 2 can be controlled by temperature. Since it becomes difficult to control to a desired solid phase ratio if the temperature range in which the solid-liquid coexistence state is too narrow, it is desirable to determine the composition so that the temperature range in which the solid-liquid coexistence state coexists is 1 ° C. or more. . In addition, when a heat storage material that is in a solid-liquid coexistence state in which substances are different between a solid phase and a liquid phase is used as in Patent Document 1, it is difficult to control the solid fraction by temperature.

  As described above, when a two-component mixed salt having a non-eutectic composition is used as the heat storage material 2, the heat storage material 2 solidified into a solid phase is the cooling surface (the surface of the heat transfer tube that is the cooling side heat exchanger 4). It does not adhere strongly to the surface and easily comes off.

  This is because the solid fraction near the cooling surface increases locally when it begins to solidify, and solidification proceeds while the molten salt in the liquid phase with a relatively low melting point enters between the cooling surface and the solid phase. It is considered that the solid-liquid coexistence region of non-eutectic molten salt exists with a temperature range. Therefore, by controlling so that the solid phase gradually increases from the liquid phase state (that is, gradually lowering the temperature), the heat storage material 2 that has become the solid phase is in close contact with the cooling surface. Can be suppressed.

  The solid-phase heat storage material 2 that has been peeled off due to the flow or stirring of the heat storage material 2 has a higher specific gravity than the liquid-phase heat storage material 2, and thus settles in the lower part of the heat storage tank 3. Therefore, by providing the cooling side heat exchanger 4 in the upper part of the heat storage tank 3, the liquid phase heat storage material 2 can be efficiently cooled. Also, by providing the heating side heat exchanger 5 on the bottom surface of the heat storage tank 3, the solid phase heat storage material 2 is pressed against the heating side heat exchanger 5 by its own weight, and the solid phase heat storage material 2 is efficiently used. It becomes possible to heat.

In the present embodiment uses a mixture of potassium nitrate and sodium nitrate as the thermal storage material 2 is not limited to this, if the mixed salt of a non-eutectic composition as the solid-liquid coexisting state at the heat storage minimum temperature T _min, heat storage It can be used as the material 2.

For example, when T _max = 400 ° C. and T _min = 250 ° C., a mixture of CsNO ₃ and NaNO _3, a mixture of LiNO ₃ and NaNO _{3, and} a mixture of NaNO ₃ and RbNO ₃ are used as the heat storage material 2. Is possible. Further, when T _min = 280 ° C., a mixture of LiBr and NaNO ₃ or the like can be used as the heat storage material 2. The respective equilibrium diagrams are shown in FIGS.

As shown in FIG. 2, when a mixture of CsNO ₃ and NaNO ₃ is used as the heat storage material 2, T _min = 250 by setting the molar fraction of NaNO ₃ to 0.902 (mass fraction of 0.801). It can be in a solid-liquid coexistence state at ° C. Similarly, when the mixture of LiNO ₃ and NaNO ₃ in FIG. 3 is used as the heat storage material 2, the molar fraction of NaNO ₃ is 0.877 (the mass fraction is 0.898), and the NaNO ₃ and RbNO _{3 in} FIG. When the mixture is used as the heat storage material 2, the solid-liquid coexistence state can be obtained at T _min = 250 ° C. by setting the molar fraction of RbNO ₃ to 0.105 (the mass fraction is 0.169). Further, when the mixture of LiBr and NaNO ₃ in FIG. 5 is used as the heat storage material 2, the solid fraction at T _min = 280 ° C. is obtained by setting the molar fraction of NaNO ₃ to 0.964 (mass fraction of 0.963). It can be in a liquid coexistence state. 2 to 5, similarly to FIG. 1B, the temperature region in which the solid-liquid coexistence state exists is indicated by a bold solid line.

As the heat storage material 2, as long as it is a non-eutectic salt mixture of the solid-liquid coexisting state at the heat storage minimum temperature T _min, it is also possible to use more than three components.

For example, a mixed salt of NaNO ₃ , KNO ₃ and NaNO ₂ called heat transfer salt (HTS) is known as a ternary molten salt. The composition of this HTS was changed from the commonly used eutectic composition (NaNO ₃ : 7 mol%, KNO ₃ : 44 mol%, NaNO ₂ : 49 mol%, melting point 142 ° C.), NaNO ₃ : 7 mol%, KNO _The solidification experiment was conducted with a non-eutectic composition of 3:70 mol% and NaNO ₂ : 23 mol%. As a result, it was confirmed that the heat storage material 2 that became a solid phase peeled off as small particles from the cooling surface, and the solid phase easily separated from the cooling surface.

As described above, in the thermal storage system 1 according to this embodiment, as the thermal storage material 2 is used one made of a mixed salt of a non-eutectic composition as the solid-liquid coexisting state at the heat storage minimum temperature T _min.

  As a result, the heat storage material 2 that has solidified to become a solid phase does not adhere strongly to the cooling surface, but easily peels off, so that it is possible to suppress a decrease in heat transfer due to the heat storage material 2 that has become a solid phase. . As a result, it is possible to use latent heat without stretching the heat transfer tube around the entire heat storage tank 3, thereby reducing the cost and improving the amount of heat storage.

Furthermore, in the heat storage system 1, since the heat storage material 2 does not adhere strongly to the cooling surface and peels off, even if the solid phase ratio at the heat storage minimum temperature _Tmin is set higher than before, a decrease in heat transfer can be suppressed. It becomes possible to further improve the heat storage amount.

In the heat storage system 1 in which the minimum heat storage temperature T _min is 150 ° C. or higher, a heat storage material 2 that can be used in a solid-liquid coexistence state and that can suppress adhesion of the solid phase to the cooling surface has been conventionally found. Therefore, it is considered that the present invention greatly contributes to the development of the heat storage system 1 used in a high temperature region.

  The present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the spirit of the present invention.

  In order to experimentally grasp whether the solid phase is fixed to the cooling surface or can be detached, an experimental apparatus 61 as shown in FIG. 6 was created and the experiment was performed.

  The experimental device 61 is configured to immerse one end portion of the metal plate 64 in the heat storage material 2 stored in the beaker 63 and to cool the other end portion of the metal plate 64 exposed from the heat storage material 2. Yes. The temperature of the heater 67 arranged under the beaker 63 is gradually lowered, and the temperature distribution in the longitudinal direction of the metal plate 64 and the temperature of the heat storage material 2 are measured by the thermocouple 65 and the data logger 66 while the solid phase is generated. Was observed. Although not shown, in order to facilitate observation, the other end of the metal plate 64 exposed from the heat storage material 2 was kept warm to slow down the generation of the solid phase.

  As the heat storage material 2, a mixture of potassium nitrate and sodium nitrate shown in FIG. For heat storage, it is advantageous to have a large specific heat. Therefore, a mixture of potassium nitrate and sodium nitrate usually has a composition containing more sodium nitrate than a composition that becomes a eutectic (sodium nitrate molar fraction is 0.49), specifically, It is used in a composition (called solar salt) in which the molar fraction of sodium nitrate is 0.64. Here, the composition shown by the thick broken line in FIG. 1 (b) has more sodium nitrate than solar salt, that is, a non-eutectic crystal having a sodium nitrate molar fraction of 0.786 (mass fraction of 0.755). Used in composition.

  When the temperature of the heat storage material 2 is gradually lowered, a solid phase is generated in the vicinity of the meniscus of the metal plate 64 having the lowest temperature and grows along the surface of the metal plate 64. The liquid phase heat storage material 2 being stirred was peeled off. Further, the solid phase grown along the surface of the metal plate 64 was translucent, dendrite-like (dendritic) irregularities were observed, and the convex portions were less transparent than the thin portions. FIG. 7A shows the temperature history at this time. As shown in FIG. 7 (a), the temperature of the metal plate 64 decreased with the temperature decrease of the heat storage material 2, but the temperature difference was substantially constant.

  In the non-eutectic composition, a region that is in a solid-liquid coexistence state exists with a temperature range, and the solid phase ratio at that temperature is uniquely determined. When solidification starts, the solid phase ratio in the vicinity of the surface of the metal plate 64 is locally increased, so that the solidification proceeds while the liquid heat storage material 2 having a relatively low melting point enters between the metal surface and the solid phase. It is done. Therefore, it is considered that the heat storage material 2 that has become a solid phase does not adhere to the metal plate 64 until the temperature is sufficiently lowered and is easily peeled off.

  On the other hand, for comparison, the heat storage material having the eutectic composition was gradually cooled and observed. A transparent thin film-like solid phase is generated on the surface of the metal plate 64 immersed in the heat storage material, the thickness gradually increases, and it is peeled off by the flow of the liquid heat storage material being stirred. There wasn't. FIG. 7B shows the temperature history at this time. As shown in FIG. 7 (b), the heat storage material maintained a substantially constant temperature during solidification, but the temperature of the metal plate 64 greatly decreased with the growth of the solid phase, and the temperature difference increased.

  As described above, when a heat storage material 2 having a non-eutectic composition is used, the solid phase generated on the metal surface is peeled off due to the flow of the liquid heat storage material 2 being stirred. The composition did not peel off. The difference in adhesion of the solid phase to the metal surface is thought to be due to the existence of a solid-liquid coexistence region of non-eutectic molten salt with a temperature range.

Therefore, as the heat storage material 2, the heat storage material 2 in the solid phase does not adhere strongly to the cooling surface by using a mixed salt of a non-eutectic composition that is in a solid-liquid coexistence state at the minimum heat storage temperature T _min . It becomes easy to peel off, and it becomes possible to suppress a decrease in heat transfer due to the heat storage material 2 in the solid phase.

DESCRIPTION OF SYMBOLS 1 Thermal storage system 2 Thermal storage material 3 Thermal storage tank 4 Cooling side heat exchanger 5 Heating side heat exchanger

Claims (3)

In a heat storage system with a minimum heat storage temperature of T _min ,
A heat storage system using a non-eutectic mixed salt that is in a solid-liquid coexistence state at the minimum heat storage temperature T _min as the heat storage material. The heat storage system according to claim 1, wherein a material having a temperature range of 1 ° C. or higher is used as the heat storage material. The heat storage system according to claim 1 or 2, wherein the minimum heat storage temperature T _min is 150 ° C or higher.

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